Crash Dynamics

A collision or crash is a random multi-factor event where one or more individuals fail to cope with their environment. It is an undesired circumstance(s) which gives rise to ill health, injury, damage, or increased liabilities.


There are many factors related to injury prevention that must be considered before, during, and after a crash to prevent or minimize injuries from occurring. Here are a few examples:

  • Road & vehicle conditions before the crash (slippery road, tire conditions, etc)
  • Car seat usage during the crash
  • Seat belt usage (such as using lap belt correctly or incorrectly) during the crash
  • Emergency response time after the crash

You can be a very good driver and drive safely with optimal vehicle condition, but still get into a collision. To understand the value of occupant protection and how it helps you survive a collision we have to look at the dynamics of a collision. Every vehicle collision includes three types of crashes.

  1. The vehicle crash
  2. The human crash
  3. The internal crash
Three types of crashes in a collision. (Image source:

The Vehicle Crash
The first stage involves the exterior of the vehicle. A crash causes the vehicle to buckle and bend as it hits something, either another vehicle or a tree and comes to an abrupt stop. This occurs in approximately one-tenth (1/10) of a second in a frontal collision. The crumple zone where the front end crushes, absorbs some crash forces and cushions the rest of the vehicle. As a result, the passenger compartment comes to a more gradual stop than the front of the vehicle.

Car crash
The vehicle crash. (Image source:

The Human Crash
The second stage occurs in the interior of the vehicle as the vehicle stops. In a crash, occupants move toward the point of impact, at the vehicle’s original speed. Just after the vehicle comes to a complete stop, occupants collide with the steering wheel, windshield, seat belt, or some other part of the vehicle interior. All objects in the vehicle move with the same speed upon impact whether restrained or not.

Image source:

Another form of the human crash is the person-to-person impact:

  • Unrestrained occupants colliding with each other or an unrestrained occupant colliding with a restrained occupant can cause many serious or fatal injuries.
  • Unrestrained rear-seat occupants become high-speed projectiles striking people in the front seat.
Unbuckled rear-seat occupant becoming projectile striking the driver. (Image source:

The Internal Crash
The third stage occurs after an occupant’s body comes to a complete stop. The internal organs are still moving forward until the organs hit something like another organ or the skeletal system. This third crash is the internal crash, often causing serious or fatal internal injuries.

An example of an internal crash happening to the brain hitting the skull. (Image source:

In any crash including a minor one, occupants of the vehicle can be seriously injured. Most people do not know the force a vehicle has when moving. The force needed to restrain an occupant approximately equals the weight of the occupant multiplied by the vehicle speed.

Crash Forces = Weight ✖ Speed = Restraining Force

Example: A 3 kg infant in a vehicle moving at 60 km/h could require at least 180 kg
(3 x 60 = 180) of restraining force to keep from moving forward.


  • A car going 50 km/h would hit a tree with the same force as hitting the ground after falling off a 3-storey building. The faster the speed, the harder the impact.
  • It is important for parents to understand that the forces involved in a crash can kill or cause serious injuries to themselves and their child.

    Image source:

Dangerous crash events can occur in almost any type of collision or chain of crash events. Here are the most common types of crashes and their related injuries.

  • Frontal crashes are the most frequent and can result in neck, head, upper body, and lower body injuries.
  • Rear-end crashes are also common and can result in back and neck injuries.
  • Lateral and side-impact crashes can result in torso, head, hip, and leg injuries.
  • A rollover crash occurs when the vehicle rolls over onto its side or top (upside down) one or more times. A vault is similar, but the vehicle flips end over end. A rollover/vault is often responsible for occupants being thrown from vehicles.
  • In a rotation (or spin), unrestrained occupants are more likely to be injured as they hit the vehicle interior repeatedly and are much more likely to be thrown from the vehicle than restrained occupants.
  • In an ejection, vehicle occupants are thrown out a window or door, skid along the pavement, and may be pinned or crushed under a vehicle. Landing gently on a soft surface is highly unlikely.

People thrown from a vehicle are four times more likely to be killed than those who remain inside the vehicle (NHTSA, 2009).

A common myth about car seat, booster seat, and seat belt use is that occupants are better off being thrown clear of a crash. Even in the very rare chance of a vehicle fire or landing in the water, a properly belted occupant is more likely to be uninjured and conscious, thus able to exit from the vehicle.

Find out more on how car seats, booster seats and seat belts help to prevent injuries and deaths here.

Edited by Dinas E. on 17th February 2018.

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